This invention relates to the production of dimethyl substituted C.sub.4 olefins and to special dialkyl glycol ethers.
Syntheses of 3,3-dimethylbutene-1 (neohexene) from 1-chloro-3,3-dimethylbutane (neohexyl chloride) are described in the literature, such as, for example, in German Pat. No. 1,253,700, suggesting a thermal dehydrochlorination at 350.degree.-800.degree. C. (500.degree.-700.degree. C.) and brief residence times in the absence of catalysts. This process is expensive owing to the high temperatures and the problems associated with the resultant, corrosive hydrogen chloride by-product. In addition, the residence time must be very accurately controlled to avoid the formation of major amounts of 2,3-dimethylbutene-1 and 2,3-dimethylbutene-2 due to rearrangement of 3,3-dimethylbutene-1 (neohexene).
Sodatova et al (see Chemical Abstr. 79: 41796 [1973]) propose the reaction of 1-chloro-3,3-dimethylbutane (neohexyl chloride) with potassium acetate in the presence of acetic acid at 180.degree.-200.degree. C., and under 22 atmospheres to obtain the corresponding acetic acid ester, and splitting the latter thermally at 500.degree. C. to obtain the 3,3-dimethylbutene-1 (neohexene) and acetic acid. This two-stage process likewise requires high temperatures and can be performed only in a pressurized apparatus.
In contrast thereto, A. Brandstrom [see Acta Chem. Scand. 13: 611-612 (1959)] describes a laboratory method for the dehydrochlorination of neohexyl chloride operating in a single stage at relatively low temperatures, 120.degree.-190.degree. C., with potassium hydroxide as the alkali and polyethylene glycol as the solvent. In this process, a homogeneous mixture is obtained with the aid of the polyethylene glycol employed. However, one drawback in this connection is that the thus-produced potassium chloride cannot be removed in an industrially simple way, e.g., by a water scrubbing step, without wastewater problems. In addition, the yield is low.
Also, syntheses of 2,3-dimethylbutene-1 and 2,3-dimethylbutene-2 have been described in the literature, such as, for example, in DAS No. 2,917,779 proposing a three-stage synthesis, starting with isovaleraldehyde. In this process, isovaleraldehyde is reacted with formaldehyde to the .alpha.-isopropylacrolein, this unsaturated aldehyde is subsequently hydrogenated to 2,3-dimethylbutanol, and the dehydration conducted thereafter yields a mixture of the isomeric olefins 2,3-dimethylbutene-1 and 2,3-dimethylbutene-2. This process requires relatively expensive chemicals and is industrially expensive because it requires many stages. Despite these disadvantages, this process, as explained in detail in the specification of DAS No. 2,917,779, is an improvement over the processes heretofore known in the literature for the dimerization of propene wherein the methylpentenes, which are difficult to separate, are always obtained as the by-product.
Besides this process, the selective hydrogenation of 2,3-dimethylbutadiene has also been suggested for the preparation of 2,3-dimethylbutene-2. This starting material is, however, difficult to obtain and thus this process is of little industrial interest.
In contrast, more interesting are methods starting with the readily accessible 2,3-dimethylbutane. This hydrocarbon is formed in the production of the musk fragrance 7-acetyl-1,1,3,4,4,6-hexamethyl-1,2,3,4-tetrahydropnaphthalene (so-called acetyl-HMT, German Pat. No. 2,457,550) as a by-product, namely at a certain stage of this synthesis, in the reaction of p-cymene with 3,3-dimethylbutene-1 (neohexene) or with 2,3-dimethylbutene-1 in equimolar amounts (DAS No. 1,035,826). The mechanism behind this interesting reaction has been described in detail by T. F. Wood and J. Angioloni in "Tetrahedron Letters" 1: 1-8 (1963).
Conversion of 2,3-dimethylbutane into the olefin mixture is possible, for example, by dehydrogenation or by chlorination and subsequent dehydrochlorination. The dehydrogenation, investigated by L. B. Fisher, M. P. Terpugova, and I. L. Kotlyarevskii [Izvest. Vostoch Filial, Akad. Nauk, SSSR 1957, No. 9.53-6; or Chem. Abstr. (1958): 12746], has the disadvantage that high temperatures of about 550.degree. C. are required, and that 2,3-diemthylbutadiene is formed as a by-product. Separation of the diene and the unreacted 2,3-dimethylbutane from the monoolefin mixture is, however, very expensive on account of their very close boiling points.
This drawback is eliminated by choosing a method wherein 1-chloro-2,3-dimethylbutane and/or 2-chloro-2,3-dimethylbutane occur as the by-products since these have a considerably higher boiling point than 2,3-dimethylbutane. The production of a mixture of this chlorine compound is simple to carry out industrially by chlorination of 2,3-dimethylbutane. However, higher-chlorinated products are also formed in minor amounts in this process. The actual problem, though, resides in the subsequent dehydrochlorination since the two isomeric monochlorine compounds show very different stabilities. According to Vives, V. C.; Kruse, C. W.; and Kleinschmidt, R. F. (Ind. Engng. Chem., Product Res. Development 8 [1969] 4: 432-435), 2-chloro-2,3-dimethylbutane, as a tertiary chlorine compound, is dehydrochlorinated readily at its boiling point (bp=112.degree. C.) with a large number of catalysts, whereas the primary chlorine compound, 1-chloro-2,3-dimethylbutane, requires temperatures of about 300.degree. C. for dehydrochlorination. For this reason, these authors propose the use of tertbutyl chloride as the chlorinating agent because only the tertiary chlorine compound is produced. Isobutane is formed from the tertiary butyl chloride, namely in equimolar amounts as a waste product.
The process is therefore expensive because of its high consumption of chemicals. In the thus-produced olefin mixture, the ratio of isomers of .alpha.-olefin/.beta.-olefin is 1:3.
In order to obtain extensive dehydrochlorination of the mixture of primary and tertiary chlorine compounds, U.S. Pat. No. 2,613,233 proposes to effect a two-stage splitting step wherein, in the first stage, temperatures are set at 120.degree.-130.degree. C. and, in the second stage, at 500.degree.-600.degree. C. This process thus is likewise relatively expensive and requires costly reactors of special steels on account of the high temperatures.
All of the conventional methods accordingly demand high temperatures, expensive technical apparatus, or costly chemicals, can be carried out only in several stages, or create problems in waste disposal.